nir: fix lowering arrays to elements for XFB outputs

nir: fix lowering arrays to elements for XFB outputs

If we have a transform feedback output like:

float[2] x2_out (VARYING_SLOT_VAR1.x, 0, 0)

which is lowered by nir_lower_io_arrays_to_elements to,

float x2_out (VARYING_SLOT_VAR1.x, 0, 0)
float x2_out@5 (VARYING_SLOT_VAR2.x, 0, 0)

We have to update the destination offset to avoid overwriting
the same value.

v2 (Jason Ekstrand):
 - Compute the correct offsets for arrays of vectors and/or doubles

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>